1. Field of the Invention
The present invention relates to a method of manufacturing a bonding substrate composed of two substrates bonded together, and more particularly to a method of manufacturing a so-called bonding SOI (Silicon on Insulator) substrate in which two silicon monocrystalline substrates, or a silicon monocrystalline substrate and an insulator substrate are bonded together via a silicon oxide film.
2. Description of the Related Art
There have been known various methods of manufacturing a bonding SOI substrate in which two silicon monocrystalline substrates are bonded together via a silicon oxide film. In the method shown, for example, in Japanese Patent Publication (kokoku) 5-46086, an oxide film is formed on at least one of two substrates; the two substrates are brought into close contact with each other with no foreign substance being interposed between the joint surfaces thereof; and the substrates are then subjected to heat treatment at a temperature of about 200 to 1200.degree. C. in order to increase joint strength.
Since a bonding substrate whose joint strength has been increased by means of thermal treatment can undergo a subsequent grinding and polishing process, the thickness of one substrate on which devices are to be fabricated can be reduced to a desired thickness through grinding or polishing in order to obtain an SOI layer for formation of semiconductor device.
However, it is known that a bonding substrate manufactured in the above-described manner has an unjoined portion in an area extending about 1-3 mm from the peripheral edge of the substrate. In order to remove such an unjoined portion, there have been developed various techniques such as those shown in Japanese Patent Application Laid-Open (kokai) Nos. 3-89519, 4-263425, 3-250616, and 64-89346.
Although these methods can remove such an unjoined portion, they have the following drawbacks. In the techniques disclosed in Japanese Patent Application Laid-Open Nos. 3-89519 and 4-263425 in which the peripheral portion of one substrate on which devices are to be fabricated (bond wafer) is ground to reach the other substrate which serves as a support substrate (base wafer), the shape of the base wafer changes greatly from the original shape. In the technique disclosed in Japanese Patent Application Laid-Open No. 3-250616 in which a piece of masking tape is applied to a wafer to cover the entire wafer except the peripheral portion, and then etching is performed in order to remove the unjoined portion at the periphery of the wafer, the process becomes complicated. In the technique disclosed in Japanese Patent Application Laid-Open No. 64-89346 in which the entire peripheral portion of a bonded wafer is removed through etching, the production requires a prolonged period of time and high costs and the productivity is low.
In order to solve these problems, there has been proposed another method of manufacturing a bonding substrate as disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-45485. In this method, an oxide film is formed on the surface of at least one of two wafers (bond and base wafers); the two wafers are brought into close contact with each other via the oxide film; the wafers are heat-treated in an oxidizing atmosphere in order to firmly join the wafers together; an unjoined portion at the periphery of the bond wafer is completely removed; and the bond wafer is ground/polished to a desired thickness, wherein the complete removal of the unjoined portion at the periphery of the bond wafer is carried in such a way that the peripheral portion of the bond wafer is first removed through grinding to a thickness such that damage does not reach the base wafer, and the unjoined portion at the periphery of the bond wafer is completely removed through etching.
The above-described methods have advantages that the shape of the base wafer is not changed, that masking tape or the like is not required to be used, and that the process does not become complicated unreasonably.
However, when the unjoined portion at the peripheral portion of the bond wafer is removed through grinding, damage may reach the base wafer unless the grinding is stopped such that the unjoined portion still has a considerably large thickness. If damage reaches the base wafer, there arises a problem that when the unjoined portion at the periphery of the bond wafer is completely removed by subsequent etching, etchant may reach the surface of the base wafer via the damaged buried oxide layer and form a scratch or depression in the surface, thereby decreasing yield in the subsequent device-fabrication step.
If the grinding amount of the peripheral portion of the bond wafer is decreased, the above-described problem can be solved. However, in this case, an amount of etching stock removal increases and therefore the etching process requires a prolonged period of time and increased costs such as that of etchant, thereby losing the advantage of mechanical grinding. From this, it may be concluded that the above-described method in which the entire peripheral edge of the bond wafer is removed through etching is preferable because of its fewer number of process steps.